The controlled expansion of gases forms the basis for the majority of non-electrical rotational engines in use today. These engines include reciprocating, rotary, and turbine engines, and may be driven by heat, such as with heat engines, or other forms of energy. Heat engines optionally use combustion, solar, geothermal, nuclear, and/or forms of thermal energy. Further, combustion-based heat engines optionally utilize either an internal or an external combustion system, which are further described infra.
Internal Combustion Engines
Internal combustion engines derive power from the combustion of a fuel within the engine itself. Typical internal combustion engines include reciprocating engines, rotary engines, and turbine engines.
Internal combustion reciprocating engines convert the expansion of burning gases, such as an air-fuel mixture, into the linear movement of pistons within cylinders. This linear movement is subsequently converted into rotational movement through connecting rods and a crankshaft. Examples of internal combustion reciprocating engines are the common automotive gasoline and diesel engines.
Internal combustion rotary engines use rotors and chambers to more directly convert the expansion of burning gases into rotational movement. An example of an internal combustion rotary engine is a Wankel engine, which utilizes a triangular rotor that revolves in a chamber, instead of pistons within cylinders. The Wankel engine has fewer moving parts and is generally smaller and lighter, for a given power output, than an equivalent internal combustion reciprocating engine.
Internal combustion turbine engines direct the expansion of burning gases against a turbine, which subsequently rotates. An example of an internal combustion turbine engine is a turboprop aircraft engine, in which the turbine is coupled to a propeller to provide motive power for the aircraft.
Internal combustion turbine engines are often used as thrust engines, where the expansion of the burning gases exit the engine in a controlled manner to produce thrust. An example of an internal combustion turbine/thrust engine is the turbofan aircraft engine, in which the rotation of the turbine is typically coupled back to a compressor, which increases the pressure of the air in the air-fuel mixture and increases the resultant thrust.
All internal combustion engines suffer from poor efficiency; only a small percentage of the potential energy is released during combustion as the combustion is invariably incomplete. Of energy released in combustion, only a small percentage is converted into rotational energy while the rest is dissipated as heat.
If the fuel used in an internal combustion engine is a typical hydrocarbon or hydrocarbon-based compound, such as gasoline, diesel oil, and/or jet fuel, then the partial combustion characteristic of internal combustion engines causes the release of a range of combustion by-products pollutants into the atmosphere via an engine exhaust. To reduce the quantity of pollutants, a support system including a catalytic converter and other apparatus is typically necessitated. Even with the support system, a significant quantity of pollutants are released into the atmosphere as a result of incomplete combustion when using an internal combustion engine.
Because internal combustion engines depend upon the rapid and explosive combustion of fuel within the engine itself, the engine must be engineered to withstand a considerable amount of heat and pressure. These are drawbacks that require a more robust and more complex engine over external combustion engines of similar power output.
External Combustion Engines
External combustion engines derive power from the combustion of a fuel in a combustion chamber separate from the engine. A Rankine-cycle engine typifies a modern external combustion engine. In a Rankine-cycle engine, fuel is burned in the combustion chamber and used to heat a liquid at substantially constant pressure. The liquid is vaporized to a gas, which is passed into the engine where it expands. The desired rotational energy and/or power is derived from the expansion energy of the gas. Typical external combustion engines also include reciprocating engines, rotary engines, and turbine engines, described infra.
External combustion reciprocating engines convert the expansion of heated gases into the linear movement of pistons within cylinders and the linear movement is subsequently converted into rotational movement through linkages. A conventional steam locomotive engine is used to illustrate functionality of an external combustion open-loop Rankine-cycle reciprocating engine. Fuel, such as wood, coal, or oil, is burned in a combustion chamber or firebox of the locomotive and is used to heat water at a substantially constant pressure. The water is vaporized to a gas or steam form and is passed into the cylinders. The expansion of the gas in the cylinders drives the pistons. Linkages or drive rods transform the piston movement into rotary power that is coupled to the wheels of the locomotive and is used to propel the locomotive down the track. The expanded gas is released into the atmosphere in the form of steam.
External combustion rotary engines use rotors and chambers instead of pistons, cylinders, and linkages to more directly convert the expansion of heated gases into rotational movement.
External combustion turbine engines direct the expansion of heated gases against a turbine, which then rotates. A modern nuclear power plant is an example of an external-combustion closed-loop Rankine-cycle turbine engine. Nuclear fuel is consumed in a combustion chamber known as a reactor and the resultant energy release is used to heat water. The water is vaporized to a gas, such as steam, which is directed against a turbine forcing rotation. The rotation of the turbine drives a generator to produce electricity. The expanded steam is then condensed back into water and is typically made available for reheating.
With proper design, external combustion engines are more efficient than corresponding internal combustion engines. Through the use of a combustion chamber, the fuel is more thoroughly consumed, releasing a greater percentage of the potential energy. Further, more thorough consumption means fewer combustion by-products and a corresponding reduction in pollutants.
Because external combustion engines do not themselves encompass the combustion of fuel, they are optionally engineered to operate at a lower pressure and a lower temperature than comparable internal combustion engines, which allows the use of less complex support systems, such as cooling and exhaust systems. The result is external combustion engines that are simpler and lighter for a given power output compared with internal combustion engines.
External Combustion Engine Types
Turbine Engines
Typical turbine engines operate at high rotational speeds. The high rotational speeds present several engineering challenges that typically result in specialized designs and materials, which adds to system complexity and cost. Further, to operate at low-to-moderate rotational speeds, turbine engines typically utilize a step-down transmission of some sort, which again adds to system complexity and cost.
Reciprocating Engines
Similarly, reciprocating engines require linkages to convert linear motion to rotary motion resulting in complex designs with many moving parts. In addition, the linear motion of the pistons and the motions of the linkages produce significant vibration, which results in a loss of efficiency and a decrease in engine life. To compensate, components are typically counterbalanced to reduce vibration, which again increases both design complexity and cost.
Heat Engines
Typical heat engines depend upon the diabatic expansion of the gas. That is, as the gas expands, it loses heat. This diabatic expansion represents a loss of energy.
Problem
What is needed is an external combustion rotary heat engine that more efficiently converts the about adiabatic expansive energy of the fuel driving the engine into rotational power and/or energy for use driving a variety of applications.